I have reproduced this problem with spurious error messages. It appears tobe caused by the fonts used in your Input cell (recreating the cell withthe default fonts does not produce the error messages). I also note thatthis appears to be related to dynamic highlighting - turning dynamichighlighting off in the Preference menu (using the appropriate checkbox inthe Interface tab) also resolves the problem.

I have filed a report with our developers on this issue so that the problemcan be fixed in a future version of Mathematica.

to which I responded:

Thank you for investigating this issue. I have a comment and a conclusion:

It is true that in the file I sent you, the font was smaller than normal.This happened during my efforts to reproduce the bug in a simple case.However the bug does occur in the "normal" font, provided that **some** ofthe text in the usage-string is reduced in size (by selecting it andpressing command-minus.) Reducing text like this is normal -- I learned todo it by copying the style Mathematica employs for its own usage statements.

For example, if you evaluate "?Part", you will see that Mathematica's usagestatement for 'Part' contains both italicized and size-reduced portions --and this evokes the bug. Therefore the bug is NOT caused by a"non-standard" font; it is caused by some change in v9 which brokeMathematica-style usage statements (when written by users). Therefore thebug might be deemed to have higher priority than otherwise.

I have thousands of such usage statements in hundreds of notebooks indozens of projects -- and I learned the style by copying Wolfram!! Withversion 9, I have learned to ignore the errors in the Messages window, asthey are 99.99% erroneous -- they appear by the dozens as I edit any of mymany notebook files.